Orthogonal frequency division multiplexing is an example of a multi-carrier transmission technique that uses symbol-modulated orthogonal subcarriers to transmit information within an available spectrum. When the subcarriers are orthogonal to one another, they may be spaced much more closely together within the available spectrum than, for example, the individual channels in a conventional frequency division multiplexing (FDM) system. To achieve orthogonality, a subcarrier may have a null at the center frequency of the other subcarriers. Orthogonality of the subcarriers may help reduce inter-subcarrier interference within the system. Before transmission, the subcarriers may be modulated with a low-rate data stream. The transmitted symbol rate of the symbols may be low, and thus the transmitted signal may be highly tolerant to multipath delay spread within the channel. For this reason, many modem digital communication systems are using symbol-modulated orthogonal subcarriers as a modulation scheme to help signals survive in environments having multipath reflections and/or strong interference.
Communication systems that use symbol-modulated orthogonal subcarrier communications may have reduced channel capacity due to multipath fading and other channel conditions. Thus, there are general needs for apparatus and methods that increase channel capacity, improve channel equalization and/or reduce the effects of multipath fading, especially in systems using symbol-modulated orthogonal subcarrier communications.